1. Field of the Invention
The present invention relates to a process for purifying ethylene glycol carbonate (EGO) containing impurities from the production or work-up process, by treatment with activated carbon.
The process makes possible, for example, the purification of an EGC stream which is bled from the reaction circuit for producing EGC for the purpose of avoiding an accumulation of byproducts.
The process allows the virtually complete recyclability of this purified EGC side-stream in the EGC production and work-up circuit and thereby contributes to a material-saving mode of operation in the production of EGC.
2. Description of the Related Art
It is known that EGO can be prepared in various ways, for example from ethylene glycol and phosgene (DE-AS (German Published Specification) 1 226 117), from ethylene oxide or ethylene chlorohydrin and carbon dioxide (Chem. Ing. Techn. 43 (1971), 903 ff; Fette, Seifen, Anstrichmittel 73 (1971), 396 ff; DE-OS (German Published Specification) 2 855 232, corresponding to U.S. Pat. No. 4,314,945; Ind. Eng. Chem. 50 (1958), 767-770; DE-OS (German Published Specification) 4,141,189, DE 4,138,438; DE 4,210,943) and also from ethylene, oxygen and carbon dioxide (U.S. Pat. No. 3,025,305). The EGC is here generally obtained in a form contaminated by the various materials from the production process and secondary components.
In the process starting from ethylene oxide and carbon dioxide, the crude product further contains the dissolved catalyst, for example quaternary ammonium compounds, alkali metal halides or alkali metal halides in combination with zinc halides. This has to be removed from the crude product prior to final purification, e.g. by fractional distillation. Thus, for example, in the process described in Chem. Ing. Techn. and Fette, Seifen, Anstrichmittel (loc. cit.), two thin-film distillations are carried out to remove the catalyst prior to the final purification. This separated catalyst can then mostly be recycled to the production reaction for EGC. In this thin-film distillation, the EGC is partially decomposed, even if mild conditions (maximum 50 mbar) are maintained. The decomposition products collect, depending on volatility, in the distillate or in the liquid phase of the distillation. The danger of decomposition of the EGC is due, in particular, to the fact that the catalyst is concentrated in the liquid phase of the distillation as a result of the pure EGC distilling off and that the degree of decomposition of the EGC is strongly dependent on the concentration of the catalyst dissolved therein. Thus, for an inlet concentration of from 0.25 to 0.5% by weight of catalyst, not more than from 90 to 95% of the EGC may be distilled off. In addition, the catalyst itself can be decomposed in this distillation process. If it is desired to recycle the liquid phase of the distillation, which represents a concentrated catalyst solution, into the production reaction, a part of the catalyst has to be discarded and replaced by new catalyst. Thus, for example, replacement of 30% of the catalyst solution is recommended. The replacement of the catalyst solution is carried out by bleeding off a part of the catalyst solution from the EGC production and work-up circuit and simultaneously feeding in the corresponding amount of fresh catalyst (Ind. Eng. Chem., loc. cit; DE 4,141,189).
In addition, the bleeding procedure avoids an accumulation of impurities, e.g. higher-boiling secondary components, which can be formed during the reaction and the work-up.
The bleed stream can, depending on the conditions of EGC production and work-up, comprise up to .gtoreq.90% of EGC and therefore represents a significant loss in yield if it has to be discarded.
This is particularly true when production and work-up are carried out under gentle conditions, for example by adiabatic preparation of the EGC as in DE 4,141,189 using relatively low pressures and low separation rates in the vacuum distillation. In such gentle production and workup, the bleed stream can even be about 99% EGC.
It was therefore desirable to have a purification process for EGC contaminated by higher-boiling secondary components and possibly laden with catalyst from the EGC production process, which purification process leads to as complete as possible removal of the secondary components but only to low catalyst losses. Furthermore, the purification process should itself represent no stress on EGC and catalyst and be energy conserving.